Never heard of triangulation? Or maybe you’ve tried the GEDmatch Triangulation Tool and just don’t get it.
This article gives a step-by-step walkthrough of triangulating DNA matches with a mix of the GEDmatch tools.
What Is Triangulation On GEDmatch?
Triangulation may seem complicated, but it’s not. It’s easiest to explain with an example.
Let’s say that you inherited a segment of DNA on Chromosome #2 passed down from your great-great-grandfather on your father’s side.
And your mother’s great-great-grandmother passed down a segment of DNA on chromosome #8.
The GEDmatch One-To-One Comparison tool shows that you share the same segment of DNA on Chromosome #8 with DNA matches Clay and Pamela.
Does that mean that these two matches are descended from your maternal 3rd great-grandmother?
Unfortunately, you can’t know with the current level of consumer DNA analysis.
The current problem with chromosome pairs
You inherit one chromosome within a pair from your father and the other from your mother. Your results don’t distinguish between individual chromosomes within each pair.
So, the shared DNA showing up for Clay may be on your paternal copy. While your shared DNA with Pamela may be on your maternal copy.
This would mean that Pamela and Clay don’t descend from the same line.
When you compare Pamela and Clay using the One-To-One Comparison tool, they don’t share any DNA with each other.
How triangulation can solve the problem (partially)
With triangulation, you find a set of DNA matches that share a piece of DNA with you and share the same piece with each other.
Let’s say that you find three more DNA matches that share the same piece of DNA on Chromosome Pair #8. Then you compare every match with each other.
The results are that Clay and Joshua share DNA with each other but not with the other three.
And the trio of Pamela, Sheila, and Michael all share the same piece of DNA with each other, but not with Clay and Joshua.
This exercise of comparing your DNA matches with each other at the segment level is called triangulation.
The goal is to identify separate groups that share the same piece of DNA with each other. Then, you can work on the basis that this piece of DNA was inherited from the same ancestor.
Why you want at least three DNA matches in a group
DNA doesn’t pass unchanged from generation to generation. Tiny pieces break, recombine, and generally shuffle around.
We don’t always share DNA with another person due to inheritance from the same ancestor.
There are two other possibilities. One is that the random shuffling of inheritance has led to the shared DNA being due to chance.
Another possibility is that the segment of DNA is prevalent across a community or ethnicity. When the common ancestor harks back thousands of years, this won’t be helpful for recent genealogy.
So, the more DNA matches you can place in a triangulated group, the less likely that your research is derailed by chance.
Triangulation Versus Shared Matches
You may be familiar with the Shared Match feature on your source DNA website.
GEDmatch has a similar free tool called Match Both Kits. We have a separate tutorial on GEDmatch shared matches.
Grouping your shared matches and looking for common ancestors is a great technique. The challenge is that you can’t know whether the shared kits descend on the same line.
Groups of matches who share DNA may do so through different lines i.e. different common ancestors.
By going down to the level of segments on chromosome pairs, you can get additional certainty that groups of matches have a common ancestor.
Do You Need To Worry About Boundaries?
Take a look at this display of overlapping shared DNA segments:
Clearly, the start and end positions aren’t aligned. Joshua and Sheila seem to have the same boundaries, with Pamela and Michael also looking fairly similar.
Does this mean that there are two different groups here, and we throw Clay away? No, not for triangulation purposes. The boundaries don’t need to be at the same position. The matches just have to overlap each other to some extent.
I’ve drawn a dotted line showing the overlapping pieces in the picture below.
Just be careful that the common section is long enough to be significant. I prefer not to work with pieces that are below 10 cM.
Disclaimer For This Worked Example
When I first read Jim Bartlett’s articles on triangulation, I had a few goes with my DNA matches. And I couldn’t see what all the fuss was about!
It eventually dawned on me that many people struggle to identify whether their matches are maternal or paternal.
I have no such problem. My father is from a country where consumer DNA testing is not “a thing”. So, 99.9% of my DNA matches are on my maternal side.
For this tutorial, I want to show how triangulation works by stepping through a real-life example. But there’s no point using my own kit. So, I’ve used an American friend’s kit instead.
I think the tutorial would get confusing if I kept referring to “her matches” or “my friend’s segments”. For the rest of the article, I’m saying “me” and “my” as if this were my kit.
Steps For Triangulation On GEDmatch
The rest of this article is a walkthrough of a real example.
I’m using the general method that I first read about on Jim Bartlett’s website. I think some of his articles pre-date the arrival of the Triangulation Tool on GEDmatch.
These are the steps that I’ll describe in detail.
- Step 1. Pick your starting DNA match
- Step 2: Identify The Common Chromosome(s) And Segment(s)
- Step 3: Find More Matches Who Share This Chromosome Area
- Step 4: Triangulate With The One-To-One Tool
- Alternative Step 4: Use the Triangulation Tool
Step 1: Pick The Starting DNA Match
Run the One-To-Many tool and pick an interesting match.
I’ve picked Joshua with whom I share 30 cM.
Why Joshua? Because the source column tells me that Joshua uploaded from Ancestry. There is a match with the same user name on Ancestry with a large tree going back many generations.
I have hopes that once I’ve completed the triangulation process, I’ll be able to use Joshua’s tree to help me research our common ancestor.
How to spot users across different sites
The estimated cMs are never the same across the sites, but the other details are similar enough to convince me that this is the same user. Basically, Joshua uses his first and last name as his user name and it’s a reasonably unusual surname.
I could also check the shared match lists on both GEDmatch and Ancestry to see if I can spot other users on both sites.
I will also mention that I’ve spotted Joshua on MyHeritage too. This is a guy who tested on Ancestry and uploaded his results to several DNA sites. That is a good sign that he may be open to communication and collaboration at a later stage.
Step 2: Identify The Common Chromosome(s) And Segment(s)
Run the One-To-One Autosomal Comparison tool on your chosen match (I have a separate tutorial on the GEDmatch one-to-one tool).
When I’m looking at the One-To-Many display, I can launch the free tier One-To-One comparison by clicking on the link in the “Largest” column.
At this point, I don’t know which chromosome(s) we share DNA on. So, I leave all the parameters at their default settings.
It turns out that Joshua and I share a segment of DNA on Chromosome Pair #2.
So, I make a careful note of the start and end positions. And I keep this browser page open for easy copy-and-paste to some other GEDmatch tools.
Step 3: Find More Matches Who Share This Area Of Chromosome
Now I’m going to use one of the Tier 1 tools.
Trust me, this is worth the ten bucks for a month’s access. If you’re not sure, keep reading this worked example. I’ve written a separate tutorial on my favorite paid GEDmatch feature: the Segment Search tool.
For this example, I set the parameters to run the report for Chromosome Pair #2. I also copy the start and end position of the segment I share with Joshua – remember, I kept the One-To-One browser page open!
Here is a truncated version of the display.
There’s Joshua in the second row, nestled in a stack of DNA matches who share the same area on the chromosome pair.
This stack of DNA matches becomes the starting point of triangulation.
I still don’t know if all six of us share the same common ancestor. That’s because I don’t know if they share DNA with each other.
Remember, the DNA tools do not separate the pairs of chromosomes.
I may share DNA with Joshua on the single chromosome I inherited from my mother. And I may share DNA with Clay on the single chromosome I inherited from my father.
This would mean that Joshua and Clay don’t share DNA with each other. (Unless my parents are related).
So, we need to move on to the next step: identify if each match shares DNA with each other.
Step 4: Triangulate With The One-To-One Tool
I’m going to show you two ways to triangulate this group of matches.
The first way uses the One-To-One comparison tool.
Although it takes a little more effort, I think you should use this route the first few times you triangulate some groups. I think it’s easier to understand when you’re new to this process. Then you can move on to the “short-cut” of the triangulation tools on GEDmatch.
To use the One-To-One tool to triangulate, you basically go into a loop of steps. It’s the same simple steps repeated for match pairs.
So, let’s crack on with my worked example.
I know I share DNA with these five matches at an overlapping area on Chromosome Pair #2. I just need to compare each match with one another.
(4.1) Start the loop with the top match
The first match in the group is Clay.
It doesn’t matter who you take first, but it’s easiest not to lose your place if you start at the top and work your way down.
I launch the One-To-One tool and enter Clay as Kit Number 1 in the parameters.
(4.1.2) Compare match #1 with match #2
Joshua is the second match on the list. So I enter Joshua as Kit Number 2 and run the One-To-One Tool.
Here’s a tip: don’t bother with the graphics for this exercise. I switch to a “Position Only” display, I find it a little faster.
When I see the results – sure enough, Clay and Joshua share DNA on our target area on Chromosome Pair #2.
They also share DNA with me on this area, so Clay, Joshua, and I are in a Triangulation Group.
That doesn’t mean that any of the other DNA matches in our target group are part of this first little gang.
Jim Bartlett likes to label his first Triangulation Group as TG A (the next will be TG B etc). Let’s go with that.
Right now, TG A has Clay and Joshua. Time to move on to check match #3.
(4.1.3) Compare match #1 with match #3
When you hit the back button on the One-To-One display page, you’ll be taken to the parameter page with the kit numbers still filled in.
This is a nice time-saver for the exercise in hand. Keep the top kit (Clay) and change Kit 2 (Joshua) for the next kit down.
In our example, I’m overwriting Kit 2 with Sheila’s kit number and running the report.
This is what I see: No shared DNA segments found.
This means that Clay and Sheila both share DNA with me on Chromosome Pair #2. But they don’t share any DNA with each other.
How can that be? Remember, GEDmatch and the other sites can’t distinguish between your paternal and maternal copies of each chromosome pair. Their shared segments with me are on different chromosomes within the pair.
In other words, one of the matches is on my maternal side and the other is on my paternal side.
But before we think too much about this, let’s move on with our triangulation exercise.
The important item here is that we now have a new Triangulation Group with myself and Sheila. This is TG B (Triangulation Group B).
Let’s keep moving on.
(4.1.4) Compare match #1 with match #4
Pamela is the next match down. When I run the One-To-One Comparison with Clay, I get no matching DNA again.
So, Pamela is not part of TG A (Triangulation Group A).
Can I assume she must be part of TG B? Well…let’s not jump the gun. I haven’t compared Pamela to Sheila yet. We’re going to step methodically through this process.
For now, I have a new group that I will call TG C.
This is what I’ve got so far:
|TG A||TG B||TG C|
(4.1.5) Compare match #1 with match #5
Clay also strikes out with the last match in the group, Michael.
So, Michael goes into new group TG D.
|TG A||TG B||TG C||TG D|
Using Clay as the Kit #1 in the parameters, I’ve worked through every other kit. Now, it’s time to change Kit #1. This time, Joshua gets to be top dog.
(4.2) Compare match #2 with match #3, #4, and #5
If this process seems like it will take forever, bear in mind that you’ll be running fewer comparisons with each round.
We’ve already compared match #2 and match#1, so I just need to compare Joshua with Sheila, Pamela, and Michael.
I’ll tell you now that Joshua shares no DNA with these three.
(4.3) Compare match #3 with match #4 and then with #5
Now, things get more interesting. I know that Sheila (match #3) doesn’t share DNA with Clay and Joshua.
But Sheila does share DNA on the target area of Chromosome Pair #2 with both Pamela and Michael.
So, I will add Sheila to groups C and D.
|TG A||TG B||TG C||TG D|
(4.4) Compare match #4 with match #5
Whew. This is the last of our comparisons. Both Pamela and Michael share DNA on the same target area on Chromosome Pair #2.
This means that Pamela joins Michael in group TG D and Michael joins Pamel in group TG C. If you’re thinking that one of these groups looks like the other, then we’ll address this in the final step (which is next!)
|TG A||TG B||TG C||TG D|
Review and consolidate Triangulation Groups
It’s clear that group TG C and TG D are the same (just ordered in reverse).
And there’s no need for Sheila to be on her own in TG B.
So, we eliminate B and combine C and D. That leaves us with these two groups representing two chromosomes within the pair:
|TG A||TG B|
I can confidently say I have a common ancestor with Clay and Joshua on one side of my family. While my common ancestor with Pamela, Sheila, and Michael is on the other side.
But which side is which? Triangulation won’t tell you that.
I’ll address what you do next after I’ve looked at the alternative route to triangulation.
Using The GEDmatch Tier 1 Triangulation Tool
In the previous section, I showed you a simple and methodical process that used the One-To-One tool to triangulate five DNA matches.
But as well as being simple, it was also very repetitive. GEDmatch has a paid Triangulation Tool that automates the steps that you took yourself.
I suggest that you launch the Triangulation Tool from the Segment Search Tool. This lets you tell the tool exactly which DNA matches to triangulate.
Alternatively, you can launch it from the menu. The parameters will let you choose a specific chromosome to evaluate. But the number of matches in the display can be overwhelming. I usually want to zero in on a small cluster of matches.
Launching the Triangulation Tool from Segment Search
Here is a truncated display of my Segment Search on Chromosome Pair #2.
I’m not showing the check box beside every DNA match that lets you mark which ones you want to work with.
In my case here, I will tick the box beside each of these five DNA matches.
The next step is to hit the big green “Multi-Kit Analysis” button. The Analysis page will open with your chosen kits pre-selected.
At this point, hit the “Visualization Options” button to get your choices. Triangulation is one of the tools on offer here.
Reviewing the Triangulation Tool display
In the picture below, I’ve truncated the Triangulation display by removing a lot of columns. I’ve also added in the red dotted line!
Every row shows two kits that are triangulated with you. In other words, they share overlapping DNA with you and each other.
My dotted line shows how two distinct groups have formed.
Joshua and Clay don’t match with the other three kits, so they form their own Triangulation Group, TG A.
But Pamela, Sheila, and Michael all pair up with each other on overlapping segments. This is a separate Triangulation Group, TG B.
As I discussed in the One-To-One exercise, this means that one group is paternal and one is maternal.
Again, we don’t know which is which. I’ll address that in the next section.
What Is The Cross Match Option?
You’ll see an option to turn on Cross Matching when running the Triangulation Tool.
Cross Matching on the Triangulation Tool is an extra check to find matches that share DNA across multiple rows.
The net effect is to give you an extra column with labeled groups. The truncated display below shows the extra TG column, with our trio of Pamela, Sheila, and Michael placed into a group labeled TG2_0.
The display has also separated the trio by assigning their shared segments with a different color.
Is the Triangulation Tool better than using One-To-One comparisons?
I’m not alone in preferring to run the One-To-One comparisons.
I find the display from the Triangulation Tool to be a bit clunky in the way the information is laid out.
However, I don’t use triangulation very often. If you take the time to get used to the tool’s display, then you may find it to be significantly faster to get through your research.
Step 5: Separate Paternal And Maternal Groups With Shared Matches
Shared matches will often give you a shortcut to separating these triangulation groups into paternal and maternal matches.
This technique relies on having one or more close DNA matches on GEDmatch that you have already know are on your paternal or maternal side.
You may be in the fortunate position of testing one or both parents. But even a second cousin will probably help. The key here is the “Matches Both Kits” tool.
In my example scenario, I could run the tool with an identified second cousin on my maternal side. My highest match across both groups is Clay at 60 cM, which is likely to have a decent amount of shared DNA with the known cousin.
Step 6: Looking For Common Ancestors
Now you’ve got one or more Triangulation Groups that you are confident share DNA with each other on the same chromosome.
Your next steps are to find the MRCA i.e. the most recent common ancestor. But how do you do that?
The answer is genealogy research. This challenge is to review and research the family trees of your matches to find where they intersect.